Electronic data processing (EDP) equipment such as main-frame computers, minicomputers, personal computers (PCs), word processors and related devices radiate electronic or electromagnetic radio frequency (RF) signals known as Tempest emissions. The term Tempest originates from the name of a government program concerned with the study and control of spurious electronic signals emitted from computers. Tempest emissions typically occur at several different frequencies within the range of a few kilohertz (KHz) to several hundred megahertz (MHz), and are often of sufficient strength and intensity to be received at distances exceeding several hundred meters. Within this reception range, it is possible, using sophisticated receiving and signal processing equipment, to receive and decode Tempest emissions to reveal the original information or data being processed by the source computer. Thus, signal emissions from computers are vulnerable to being received at considerable distances and decoded by unauthorized persons possessing appropriate equipment. Since computers are frequently used to process information that must be kept secret for defense, political or commercial reasons, such signal emissions are of concern to those wishing to protect sensitive data.
Awareness of the ability to remotely receive and decode computer signal emissions in order to obtain computer data has existed since the early 1960's. Both federal government agencies and private entities have expended considerable effort in attempting to protect computer data from unauthorized access through such signal emissions. One method used to protect computer data is to employ electromagnetic shielding which reduces the level of the signal emissions emanating from the computer, thus preventing effective reception at reasonable distances. Such shielding typically takes the form of a metal shield which encases the individual components of the computer, particularly the the central processing unit (CPU), video display unit (VDU), and all interconnect cables. The shield is used in conjunction with an effective AC power-line filter. The AC power-line filter is used to prevent signal emissions generated by the computer from being conducted along the AC power line and thereafter being intercepted by what is referred to as the "AC conduction mode." Where multiple, co-located computers require protection, they are often placed within a shielded-room or "vault" which incorporates a specially-designed shielded door that must be kept closed when sensitive information is being processed.
The available arrangements for shielding EDP equipment are unsatisfactory for many reasons. Shielded rooms are expensive to build and require considerable time for installation. As a practical matter, the doors of shielded rooms are seldom kept closed because of inconvenience factors and because they are difficult to open and close as a result of their weight and the close tolerances needed for an effective RF seal. Individually shielded computers are relatively expensive when compared to those without shielding. Further, certification-testing is required to verify the effectiveness of the shielding to contain the signal emissions. The sensitivity of antennas and receivers of the type that might be employed to receive computer signal emissions have improved sufficiently to render ineffective most available shielding arrangements.
Other attempts to prevent data detection from computer signal emissions involve generating a sufficiently intense, broad band, random-noise type of emission in close physical proximity to the computer in an effort to mask the signal emissions. However, these configurations typically violate Federal Communications Commission (FCC) regulations pertaining to stray radio frequency emissions, and further provide little protection against the unauthorized recovery of data from computer signal emissions detected using sophisticated receiving and advanced signal-processing technologies.